The present invention relates to a method and apparatus communicating radio frequency signals, particularly digitally encoded voice signals, with a reduced susceptibility to successful jamming. More particularly, the invention relates to a method and apparatus for overcoming jamming of voice radio communications by non-excisable interferers and by excisable interferers.
The need for reliable radio communications systems has increased in recent years, especially for military applications. As the need for reliable radio communications has increased, the sophistication and complexity of systems designed to reduce that reliability, such as jamming, have also increased. In response to the development of sophisticated jamming systems various antijamming methods and devices have been developed to restore the reliability of the radio communications.
Jamming techniques may be characterized as excisable or non-excisable. Jamming which is excisable is a relatively narrowband with respect to the desired signal. Typically, a filter may be continuously applied to excise or remove the jamming, even when such jamming is not present.
Jamming which is not excisable with a narrowband filter is termed nonexcisable. Nonexcisable jamming may come in short bursts of varying lengths on frequencies that may also vary. Nonexcisable jamming may also be masked or removed but first must be identified.
Identification of nonexcisable jamming is particularly difficult because it may be random in time and frequency or it may be spectrally similar to the desired signal. The problem of dealing with nonexcisable jamming is further complicated by the unpredictability of such jamming. Even if a particular nonexcisable jamming technique is identified the identification may not be useful to predict future jamming and must be continuously updated.
While various techniques for dealing with excisable jamming have been developed none are able to detect such jamming and apply appropriate filters only when the jamming has been detected, and none provide the vigorous and continuous identification of jamming necessary to deal with nonexcisable jamming. For example, various frequency hopping and spread spectrum techniques are described in U.S. Pat. Nos. 4,914,699 and 4,761,796 to Dunn, et al., U.S. Pat. No. 4,653,068 to Kadin and U.S. Pat. No. 4,843,612 to Brusch, et al.
In the present invention, a radio communication signal, preferably one that is a digital voice signal, is interleaved and transmitted with error correction signals related to the voice signal. The transmission may use frequency hopping and may be divided into blocks of data, called hops herein, with synchronization signals between hops. The signal received at the receiver portion of the system may contain excisable and/or nonexcisable jamming. The excisable jamming may be removed with an adaptive filter that may be applied only when the jamming is present. The nonexcisable interference may be identified and dealt with by (1) assuming different particular jamming patterns, (2) decoding to resolve the received signal for each of the assumed jamming patterns, and (3) voting all the resolved signals to determine which assumed jamming pattern produces the best results. The received signal may be demodulated using the jamming pattern which produces the best results.
Accordingly, it is an object of the present invention to provide a novel method and apparatus with an improved antijamming technique for radio communications.
It is a further object of the present invention to provide a novel method and apparatus for identifying nonexcisable jamming in a digitized voice signal.
It is yet a further object of the present invention to provide a novel method and apparatus for identifying and overcoming nonexcisable jamming on a real time basis.
It is still a further object of the present invention to provide a novel method and apparatus for determining when excisable jamming is present and applying a filter responsively.
It is another object of the present invention to provide a novel method and apparatus for proposing alternative jamming patterns and voting to determine which of the jamming patterns produces the best results without perceptively delaying reception of the signal.
These and many other objects and advantages will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.